Modern Systems Analysisand Design

Learning ObjectivesUnderstand logical process modeling via data

flow diagrams (DFDs).Draw DFDs of well structured process models.Decompose DFDs into lower-level diagrams.Balance high-level and low-level DFDs.Explain differences between current physical,

current logical, new physical, and new logical DFDs.

Use DFDs for analyzing information systems.Explain use cases and use case diagrams.

Process Modeling

Graphically represent the processes that capture, manipulate, store, and distribute data between a system and its environment and among system components

Utilize information gathered during requirements determination

Processes and data structures are modeled

Process Modeling (cont.) Deliverables and Outcomes

Context data flow diagram (DFD) Scope of system

DFDs of current physical and logical system Enables analysts to understand current system

DFDs of new logical system Technology independent Show data flows, structure, and functional

requirements of new systemThorough description of each DFD component

Data Flow Diagram (DFD)

A picture of the movement of data between external entities and the processes and data stores within a systemDifference from system flowcharts: DFDs depict logical data flow independent

of technology Flowcharts depict details of physical

systems

DFD Symbols

DFD Symbols (cont.)

Process: work or actions performed on data (inside the system)

Data store: data at rest (inside the system)

Source/sink: external entity that is origin or destination of data (outside the system)

Data flow: arrows depicting movement of data

DFD Diagramming RulesProcess

No process can have only outputs or only inputs…processes must have both outputs and inputs.

Process labels should be verb phrases.

DFD Diagramming RulesData Store

Data store labels should be noun phrases.

All flows to or from a data store must move through a process.

DFD Diagramming RulesSource/Sink

Source and sink labels should be noun phrases.

No data moves directly between external entities without going through a process.

Interactions between external entities without intervening processes are outside the system and therefore not represented in the DFD.

DFD Diagramming RulesData Flow

Bidirectional flow between process and data store is represented by two separate arrows.

Forked data flow must refer to exact same data item (not different data items) from a common location to multiple destinations.

DFD Diagramming RulesData Flow (cont.)

Joined data flow must refer to exact same data item (not different data items) from multiple sources to a common location.

Data flow cannot go directly from a process to itself, must go through intervening processes.

DFD Diagramming RulesData Flow (cont.)

Data flow from a process to a data store means update (insert, delete or change).

Data flow from a data store to a process means retrieve or use.

Data flow labels should be noun phrases.

Functional Decomposition

An iterative process of breaking a system description down into finer and finer detail

High-level processes described in terms of lower-level sub-processes

DFD charts created for each level of detail

DFD Levels

Context DFD Overview of the organizational system

Level-0 DFD Representation of system’s major processes at

high level of abstraction

Level-1 DFD Results from decomposition of Level 0 diagram

Level-n DFD Results from decomposition of Level n-1 diagram

Context Diagram

Context diagram shows the system boundaries, external entities that interact with the system, and major information flows between entities and the system.

NOTE: only one process symbol, and no data stores shown.

Level-0 DFD

Level-0 DFD shows the system’s major processes, data flows, and data stores at a high level of abstraction.

Processes are labeled 1.0, 2.0, etc. These will be decomposed into more primitive (lower-level) DFDs.

Level-1 DFD

Level-1 DFD shows the sub-processes of one of the processes in the Level-0 DFD.

This is a Level-1 DFD for Process 4.0.

Processes are labeled 4.1, 4.2, etc. These can be further decomposed in more primitive (lower-level) DFDs if necessary.

Level-n DFD

Level-n DFD shows the sub-processes of one of the processes in the Level n-1 DFD.

This is a Level-2 DFD for Process 4.3.

Processes are labeled 4.3.1, 4.3.2, etc. If this is the lowest level of the hierarchy, it is called a primitive DFD.

DFD Balancing

The conservation of inputs and outputs to a data flow process when that process is decomposed to a lower levelBalanced means: Number of inputs to lower level DFD equals

number of inputs to associated process of higher-level DFD

Number of outputs to lower level DFD equals number of outputs to associated process of higher-level DFD

Unbalanced DFD

This is unbalanced because the process of the context diagram has only one input but the Level-0 diagram has two inputs.

1 input

1 output

2 inputs

1 output

Balanced DFD

These are balanced because the numbers of inputs and outputs of context diagram process equal the number of inputs and outputs of Level-0 diagram.

1 input

3 outputs

Balanced DFD (cont.)These are balanced because the numbers of inputs and outputs to Process 1.0 of the Level-0 diagram equals the number of inputs and outputs to the Level-1 diagram.

1 input

4 outputs

Data Flow Splitting

A composite data flow at a higher level may be split if different parts go to different processes in the lower level DFD.

This remains balanced because the same data is involved, but split into two parts.

More DFD Rules

Four Different Types of DFD

Current Physical Process labels identify technology (people or

systems) used to process the data. Data flows and data stores identify actual name of

the physical media.

Current Logical Physical aspects of system are removed as much

as possible. Current system is reduced to data and processes

that transform them.

Four Different Types of DFD (cont.)

New Logical Includes additional functions Obsolete functions are removed Inefficient data flows are reorganized

New Physical Represents the physical implementation of

the new system

Guidelines for Drawing DFDs

Completeness DFD must include all components

necessary for system. Each component must be fully described in

the project dictionary or CASE repository.

Consistency The extent to which information contained

on one level of a set of nested DFDs is also included on other levels.

Guidelines for Drawing DFDs (cont.)

Timing Time is not represented well on DFDs. Best to draw DFDs as if the system has

never started and will never stop.

Iterative Development Analyst should expect to redraw diagram

several times before reaching the closest approximation to the system being modeled.

Guidelines for Drawing DFDs (cont.)

Primitive DFDs Lowest logical level of decomposition Decision has to be made when to stop

decomposition

Guidelines for Drawing DFDs (cont.)

Rules for stopping decomposition When each process has been reduced to a

single decision, calculation or database operation

When each data store represents data about a single entity

When the system user does not care to see any more detail

Guidelines for Drawing DFDs (cont.)

Rules for stopping decomposition (continued) When every data flow does not need to be split

further to show that data are handled in various ways

When you believe that you have shown each business form or transaction, online display and report as a single data flow

When you believe that there is a separate process for each choice on all lowest-level menu options

Using DFDs as Analysis Tools

Gap Analysis The process of discovering discrepancies

between two or more sets of data flow diagrams or discrepancies within a single DFD

Inefficiencies in a system can often be identified through DFDs.

Using DFDs in Business Process Reengineering

Before: Credit approval process required six days

After: process 100 times as many transactions in the same time

Use Cases

Depiction of a system’s behavior or functionality under various conditions as the system responds to requests from users

Full functioning for a specific business purpose

UML Use Case Diagram Symbols

Use Case

Actor

Boundary

Connection

Include relationship

Extend relationship

<<include>>

<<extend>>

What is an Actor?

Actor is an external entity that interacts with the system.

Most actors represent user roles, but actors can also be external systems.

An actor is a role, not a specific user; one user may play many roles, and an actor may represent many users.

What is a Boundary?

A boundary is the dividing line between the system and its environment.

Use cases are within the boundary.

Actors are outside of the boundary.

What is a Connection?

A connection is an association between an actor and a use case.

Depicts a usage relationship

Connection does not indicate data flow

What is an <<extend>> Relationship?

A connection between two use cases

Extends a use case by adding new behavior or actions

Specialized use case extends the general use case

What is an <<include>> Relationship?

A connection between two use cases

Indicates a use case that is used (invoked) by another use case

Links to general purpose functions, used by many other use cases

Written Use Cases

Document containing detailed specifications for a use case

Contents can be written as simple text or in a specified format